1. Field of the Invention
The present invention concerns a treatment process of treatment to improve the corrosion resistance and friction properties of ferrous metal parts simultaneously, and the resulting treated ferrous metal parts.
To confer both friction properties and corrosion resistance on ferrous metal parts, two distinct successive treatments are generally performed, a first treatment to give the parts the friction properties, and a second surface treatment to ensure the corrosion protection, the latter, for example, by making a deposit of zinc followed by chromating.
The friction and corrosion resistance properties acquired by these treatments are often sufficient for conventional parts.
However, for certain applications in which increasingly superior technical performance is now demanded of the parts, especially for parts subject to severe loads involving several factors simultaneously (friction, wear, especially abrasive wear, impact, corrosion), the properties conferred by conventional processes are insufficient.
This applies, for example, to parts intended for lock mechanisms, certain types of bolt and precision screw, machine screws, hinge pins, jack and shock absorber rods, and game balls or bowls. The industries concerned are in particular the automobile industry, public works, materials handlings, capital goods, home appliances and hydraulic equipment.
2. Prior art
It is known that good friction properties and good corrosion resistance can be conferred on ferrous metal parts by nitriding followed by oxidation.
This is achieved by known processes of nitriding ferrous metal parts, in particular nitriding in baths of molten cyanates and carbonates as described in French patent application 2 171 993 and French patent application 2 271 307, and nitriding in an ionized nitrogen atmosphere, which serve to improve the friction properties of these parts, by reducing the friction coefficient, and by increasing the resistance to wear and to seizure.
It is also known that if a previously nitrided part is subjected to oxidation, changes are caused in the nitrided surface and the corrosion resistance of these parts is thus improved, while preserving the friction properties acquired by nitriding. For this purpose, French patent application 2 525 637 describes a particularly effective oxidation treatment in a bath of molten oxidizing salts. Comparable results of corrosion resistance are obtained by a process of oxidation in ionized atmosphere of a gas containing oxygen.
However, this type of process confers, on the nitrided and then oxidized parts, friction properties and corrosion resistance which are insufficient for the applications mentioned.